Incandescent electric lamp



Patented; Apr. 12,- v1927.

UNITED s'ra'ras PATENT oFFlcs.

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My invention relates to the manufacture of incandescent electric lamps and more particularly to improving the quality of such lamps.

Incandescent electric lamps, especially of the yacuum type, having substantially pure tungsten filaments, are considerably improved for commercial usa e' by an operation known in the art as ashing. Briefly stated, this operation consists in lighting the lamp on a high resistance, high volta e circuit so designed that when the lamp is first lighted on this circuit a harmful flow of current will not take place within the lam By this preliminary treatment, the lamp is rendered fit for usage on a commercial circuit without danger of its destruction by aremg.

. The accepted meaning of the term arcing as employed in the lamp industry, is the conduction of the current in the space surrounding the filament and'the leading in wlres. An excessive amount .of such conduction would aflect the lamps to such an extent as to harm the normal operation thereof, and

usually results in the complete destruction of the lamp.

According to modern electrical theories, the conduction of the current in the space surrounding the filament and the leading-in wires takes place because of the presence therein of ions, i. e. charged particles which may consist of single electrons, charged atoms, or charged atom groups. to these theories, gas or vapor consisting o uncharged atoms or molecules will not .conduct any current. However, under potential gradients very much greater in value than those which may exist in normally operating incandescent-filament lamps, gaseous molecules become ionized even between cold electrodes. --It is also true that under the potential gradients existing in normally operating incandescent-filament lamps, the

" electrons emitted by the filament will not cause the conduction of appreciable current in the evacuated lamp space. But when gaseous molecules and atoms arising from the decomposition of gas evolving substances or the interaction between the filament'and such substances, at certain favorable pressures, ranging from a few thousands bars 'to about 100 bars, a bar being equivalent to about .00075 millimeters of mercury, surround incandescent filaments emitting elec- Accordin Application fled Dccember a, 1021. Serial no. name.

trons, the potential gradient in normal opcrating lam s (especially those operating above 100 V5 is sufiicient to cause the ro- (luct on of ions by the collision of rapidly moving electrons with the atoms or molecules. A sufiicient number of ions may then be present to allow the conduction of a harmful amount of current through the space surrounding the filament.

the filament temperature upon certain substances, known in the art as getters. These substances are introduced into the lamp to lmprove the efiiclencyof the lamp by removing residual gases and retarding theblackening of the bulb. An example of such getters is a mixture of an inorganic halogen compound and phosphorous held on the filament by a binder of nitro-cellulose. When a lamp containing a filament coated w th such a getter is first lighted, the number of molecules in the space surrounding the filament is considerably increased by the molecules resulting from the decomposition of the binder and the vaporization of the phosphorus and the inorganic halogen compound. During this first lighting, the getter is said to be activated. In the example given, this activation consists in vaporizing it from the filament to the bulb, for during this transitory movement it encounters the molecules of residual gas and carries them to the bulb walls and disposes the inorganic halogen salt in a transparent coating capable of retarding obscuration of the bulb by the evaporated particles of the filament. From the foregoing, it' is evident in lamps as at present constructed that (luring activation of the getter or duringfirst lighting of the lamp, pressures exist which fall within the limits set forth above, and destruction of the lamp will result if it is lighted on a commercial circuit without resistance in series therewith to prevent the conduction of a harmful amount of current.

A specific example may be helpful in understanding the mode of treatment pursued present time in the larger lamp manufacturing establishments. A 40 watt, 110 volt Mazda B lamp, which, as is well known, is anevacuated lamp having a substantially pure drawn-tungsten filament coated with a getter usually comprising a mixture of cryolite and phosphorus aflixed to the filament by a nitro-cellulose binder, is connected to a 330 volt circuit containing a resistance in series with the lamp somewhat greater than 550 ohms. This resistance, when the circuit is closed, is designed to prevent the conduction of a harmful amount of current between the leading-in wires and the filamentary parts. It is evident that even if sufiicient ions are present in the space surrounding the filament to conduct a current sufiicient to destroy the lamp, if-burned on a commercial circuit, that on the circuit described no matter how low the resistance of the lamp, a current greater than .6 of an ampere, i. e., the applied potential of 330 volts divided by the resistance of 550 ohms, cannot flow through the filament and through the space surrounding the filament, and accordingly 'the current conducted by the gas will be much less than .6 of an ampere. The flashing operation may continue for about one minute, and during this time the series resistance is successively replaced by smaller resistances until after removal of the residual gases sufficient current is allowed to flow through the filament to raise its temperature slightly above that at which it is adapted to normally operate. During this flashing operation, the phenomena of the blue glow occurs and is generally considered to indicate a successful removal of residual gases. As a result of this treatment the strength of the filament is reduced to the extent-that occasionally lamps are produced which will not withstand the shocks incident to transportation. which obviously results in a direct loss to the manufacturer as well as an inconvenience to the purchaser. This reduction, according to some authorities, is the result of the attending heat treatment received by the filament.

Having in mind that it is probably the heat treatment just described, to which practically all lamps are at present subjected, which produces the weakened condition in the filaments, I have discovered by actual experiments, more fully referred to hereinafter, that such treatment may beconsiderably modified. or' even entirely eliminated, by reason of which I have materially improved the qualities of the lamp and overcome the results incident to heat-treatment under present practices. Furthermore, I have also discovered that the shrinkage resulting directly from arcing and the reduced filamentary strength resulting from the factory methods pursued in order to prevent the occurrence of arcs when the lamp is burned directly on a commercial circuit, may be entirely eliminated.

It, therefore, becomes an object of my invention to improve the quality-of an incandescent lamp by ellccting a reduction or elimination of the harmful effects resulting. from the flashing or heat treatment given a lamp prior to shipment by the manufacturer thereof.

' In determining to what extent the treatment could be modified to secure the desired ends, I conducted a series of experiments ments gettered with cryolite, which is one of the standard getters. The getter was afiixcd to the filaments in accordance with the method described in my application for patent, Serial No. 470,672, filed May 18, 1921, for a. getter and method of applying the same and assigned to the Westinghouse Lamp Company. This method, briefly stated, differs from previous methods of applying the getter in that the getter substance is fused on the filament without the presence of organic compounds or phosphorus. These lamps were placed directly on a normal commercial circuit without having been previously burned. The lamps burned without the conduction of any harmful amount of current between the metal parts of the lamp,'and indicated in most cases that insuflicient current flowed through the space between the metallic parts of the lamp to show even the blue glow well known in the art. 'Despite contrary opinions, the vacuum obtained in such lamps as evidenced by the usual tests, was substantially equal to those in lamps in which the blue glow occurred. 7

A series of experiments were then performed in which I used a getter consisting of powdered glass, which, according to L. Hamburger in the Proceedings of the Amsterdam Academy of Sciences, vol. 21, p. 1074, possesses both the capacity of removing residual gases and nullifies the obscuration of the bulb by tungsten or filamentary 1 particles. This getter substances was affixed to the filament by the fusion process described in my copending application identified above. These lamps, without previously having been burned, were burned directly on a normal commercial circuit without any appreciable resistance in series, and none of them arced.

From the foregoing experiments I con cluded that the manner in which the getter substance is applied to the filaments has considerable to do with the prevention of arclng, provided good commercial exhaust is given the lamps. This is evident from the aforementioned experiments, as by fusing .80 with a number of lamps containing fila the getter to the filaments the introduction of substances capable of evolving or which tend to promote ionization of the gases within the lamp or which are favorable to ionization, i. e., have a low ionizing potential, maybe eliminated. Phosphorus and organic materials tend to remove any thin coating ofoxide on the filament as well as to reduce the thorium oxide in the filament with the formation of' thorium, a

metal known to have a high capacity for emitting electrons, and thus allow greater freedom of electron emission. By thus reducing the causes which promote arcing, as ex lamed more fully above, one of which being the introduction of gas-evolvingsubstances with the getter, the flashing schedule to which the lamps are subjected in order to avoid this phenomenon occurring when first lighted on a commercial circuit, may be safely eliminated.

. I further determined from a series of experiments conducted with several lamps getter'ed with cryolite and phosphorus atlixed to the filaments thereof with a nitrocellulose binder, which is in accordance with standard practice, which lamps were exhausted according to standard factory methods and thereafter flashed at 86% of tam modern theorists, I 'find it unessential to 95 the normal operating current which had been determined by experiment to be the minimum current for conditioning the lamp to burn on a commercial circuit, that no improvement was noticeable in the strength of the filament. However, in following up this line of experiments, I replaced the cryolite in the standard point getter given by a low-boiling getter substance, such as lithium fluoride, giving the lamps a slightly better exhaust than that obtained onthe automatic machines at present in use in the factory. When thus treated, I found that during the flashing operation I could safely lower the flashing current employed from 86% of normal current to a current not greater than 7 2% of normal current. Bump tests conducted on these lamps after flashing at the 72% current, showed a marked increase in filament strength over that obtained with the lamps flashed at the 86% current. From these experiments I am inclined to believe that either the improved exhaust given the lamp or the lower melting-point getter used, which probably effected a removal of residual gases under conditions of voltage and filament temperature unfavorable to gaseous conduction of electrical current, or perhaps the combination of both these changes enabled me to lower materially the flashing current. Getter substances having high ionizing potentials are to be preferred. 7

It is believed that this is a property of lithium fluoride.

Later experiments were carried out with aseries of lamps'gettered with a cryolitephosphorus-nitrocellulose binder mixture; hese lamps were given'a sufiiciently good exhaust resulting in a final pressure of a few bars and involving a heating of the lamp for one minute at a temperature just insufiicient to cause harmful softening of' the glass lamp parts at substantially atmospheric pressure immediately before transerence to the. exhaust apparatus. It'is probmost of the p osphorus and the uses resultlng from the nitro-cellulose binder, as wellas any gases held by the finely divided cryollte, were removed from the lamp.

These experiments confirmed my belief that i by removin the media which are favorable to arcing, t e lamp could be safely burned for the first time directly on a commercial circuit of the designed lamp voltage.

From the foregoing experiments and the wires, or by reducing such conductivity to a harmless amount, that the lamp may be rendered fit for safe initial burning. on a low resistance commercial circuit.

Furthermore, in contradistinction to certreat the filaments of incandescent electric lamps before burning them on a commercial circuit in order to render them safe, and am' convinced that even if such treatment if found desirable for inspection reasons or otherwise that the lamp can be burned in the factory at much reduced initial burning values, and if for any reason, it is desired to elnnlnate the present step of flashing in the fabrication of an incandescent lamp that 1 this may 'be done and the lamp safely burned for the first time directly on a commercial circuit, at which time the getter substance maybe activated.

I claim to have discovered that the electrical conductivity of the space 'surr0unding the leading-m wires and the filaments may be controlled by regulating the composition and physical state of the getter material applied to the filament when the lamp has received a good commercial exhaust. Where the vacuum in a lamp is that obtained under the best operatingexhaust conditions and substances capable of evolving gases or evaporating from the incandescent filament are not present in the lamp 1n harmful quantity, danger of arcing therein when the same is placed on a commercial'circuit need not be feared. It will thus be seen that. the preliminary flashing now accorded the lamp could be eliminated fromthe present steps pursued in the fabrication of a lamp and the cost of production correspondingly diminished. Furthermore, a lamp would be obtained whose filaable that durin the exhausting of the lamp,

. filament mentary strength is substantially unchanged and. therefore, since the original structure before burning is much stronger than that which obtains after burning, lamps could be shipped and handled without any danger of shrinkage from that source.

I also claim to have discovered that if,

for any reason, it is undesirable to eliminate flashing, as .for instance, where afinal inspection is preferred, that the original strength can be substantially maintained by reducing the lighting schedule or the current utilized in such schedule to a value where the treatmcntaccorded the lamp at that schedule will not be harmful to the strength of the filament.

These experiments show that by employing a getter having a low boiling-point, the object of flashing may be attained even in the presence of harmful gas-evolving substances at a temperature sufliciently low to materially reduce 'harmful' effects to the filament.

I am aware of certain efiorts which have been made in the past to eliminate the 'conduction of current in the space surrounding the metallic lamp parts or the so-called Edison efiect in incandescent electric lamps,

' but such efforts have not been extended to the prevention of such conduction when the "acuurn type'lamp is first lighted. On the contrary, however, this blue glova has been considered beneficial and serves as an indication of the removal of residual gases.

1 am also aware of published articles in of the lamp and applying a getter to the filament which is activated on initial burning of the filament without evolving suflicient gases to produce appreciable positive lonization.

ill

2. The method of producing an incandescent electric lamp for commercial use without previously flashing the same, which consists in reducing the electrical conductivity of the space surrounding the leading-in wires and the filament by controlling the quantity and condition of the conducting media within such space by fusing the getter to the filament in said lamp.

3. An incandescent electric lamp of the vacuum type having a low residual gas content and an unactivated getter which on initial burning of the filament iscompletely vaporized without the evolution of a harmt'ul amount of permanent gas.

4. A commercially complete incandescent electric lamp of the vacuum type having an unburned filament therein.

5. A commercially complete incandescent electric lamp of the vacuum type having an unburned filament and an unactivated get-' ter therein.

6. A commercially complete incandescent electric lamp of the vacuum type having an unburned filament and an unactivated getter thereon capable of activation when the lamp is initially lighted upon a commercial circuit.

7. An incandescent lamp having a getter substance therein which on initial burning of the lamp is vaporized without the evolution of gases which harmfully conduct the current.

In testimony whereof, I have hereunto subscribed my name this 1st day. of December, 1921.

DUNCAN MACRAE. 

